Helical three-stage isotope separation

ABSTRACT

An isotope separator of the type having an evacuated tank, a magnetic field through the tank, and an ion source and a receiver wherein the ion source is orientated at a slight angle to the magnetic field lines such that the stream of ions exiting from the ion source spiral along the magnetic field in a helical path, producing a three-stage separation after 540* of rotation, and the desired isotopic ions are collected in a receiver located at the 540* focus.

United States Patent Bell, Jr. et al.

HELICAL THREE-STAGE ISOTOPE SEPARATION Inventors: William A. Bell, Jr.;Ray L. Johnson, Jr.;

Allen M. Veach, all of Oak Ridge, Tenn.

Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission Filed: Aug. 13, 1970 Appl. No.: 63,456

[1.8. CI ..250/4l.9 ME, 250/4l.9 C

Int. Cl. ..H0lj 39/34 Field ofSearch.... .....250/4l.9 ME,4l.9 TF,4l.9C

References Cited UNITED STATES PATENTS 7/1955 McLaren ..250/4l.9 TF1/1955 Goudsmit ..250/41.9 TF

[ 1 Mar. 14, 1972 OTHER PUBLICATIONS Mass Spectromitry and itsApplication to Organic Chemistry, J. Beynon, Elsevier Publishing Co.,New York, 1960 pp. 20, 21, 37.

Primary Examiner.lames W. Lawrence Assistant Examiner-C. E. ChurchAttorney-Roland A. Anderson [57] ABSTRACT tion, and the desired isotopicions are collected in a receiver located at the 540 focus.

9 Claims, 8 Drawing Figures VACUUM PUMP PATENTEUMAR 14 I972 RECEIVERSHEET 1 (1F 6 MnM ATTORNEY.

PATENTEBHARM I972 64S 827' sum 2 [IF 6 INVENTORS.

William ABe/I, Jr. Ray L. Johnson, Jr. BY Allen M. Veach ATTORNEY.

PATENTEDMARM 1972 3,649,827

INVENTORS. William A. Bell, Jr.

Ray L. JohnsomJr. Allen M Veach ATTORNEY.

PAIENTEDMAR 14 1972 ION EXTRACTION SLIT16 SHEET a [1F 6 MAGNET MAGNETSHIMS ION 29 SOURCE POLES a IN VENTORS. William A. Bell, Jr.

Ray L. Johnson,Jr. BY Allen M Veach ATTORNEY.

PATENTEDMARMIBYZ 3,649,827

SHEET 5 [IF 6 I I I I l 130 12s 126 125 124 122 TELLURIU'M INVENTORS.

William ABe/l, Jr. Ray L. Johnson, Jr. BY Allen M. Veach A TTORNEY.

PATENTEBMAR 14 1972 3,649,827

sum 5 [IF 6 I l I I I SULFUR Fig.6 INVENTORS.

William A. Bell, Jr.

Ray L. Johnson, Jr.

BY Allen M. Veach TTORNEY.

I-IELICAL THREE-STAGE ISOTOPE SEPARATION BACKGROUND OF THE INVENTION Theinvention described herein was made in the course of, or under, acontract with the United States Atomic Energy Commission.

In any isotope enrichment process, the first two concerns are completeseparation of the various elemental nuclides and quantity of productseparated. The calutrons now in use are routinely employed to separateisotopes in large quantities. Certain of the separated isotopes arefurther enriched by another separation cycle, the product of the firstseparation being used as the feed material for the second-passseparation. In an attempt to achieve a more perfect separation, asectortype instrument, similar to those used at many separation sites,has recently been constructed.

Other separation sites employ multistage isotope separation instruments.Those devices may utilize a velocity selector (electrostatic separator)as one of the stages to obtain the most complete separation. Such avelocity selector limits throughout so that, in order to get useablequantities of product in addition to high purity, each of the stages isusually provided with its own individual magnet sector and individualmagnet supply. Thus, there exists a need for a multistage isotopeseparation device that can be economically constructed by utilizing onlya single magnetic field while at the same time providing a substantialimprovement in the purity of the final product. The present inventionwas conceived to meet this need in a manner to be described hereinbelow.

SUMMARY OF THE INVENTION It is the object of the present invention toprovide a multistage isotope separation device in which increased purityof product in addition to quantity can be achieved while using only asingle magnetic field.

The above object has been accomplished in the present invention byproviding a three-stage isotope separator which involves theacceleration of desired ions along a single homogeneous magnetic fieldas well as across it such that the ion beam from an ion source isallowed to spiral along the magnetic field in a helical path to producea three-stage separation before arriving at the 540 position. In such anarrangement, the ion source is located off the median plane close to onetank wall and the collector is likewise off center and located close tothe other tank wall. The ions pass through an angular rotation of 540before being collected and undergo separative action each 180 in amanner to be described hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an isometric view of theisotope separation device of the present invention illustrating onlypartially some of the components thereof;

FIG. 2 is a schematic drawing of the isotope separation system using thedevice of FIG. 1 and illustrating the principles of the presentinvention;

FIGS. 3A, 3B, and 3C are respective side views of the system of FIGS. 1and 2 illustrating the separative action for each 180 thereof;

FIG. 4 is a drawing showing one arrangement of magnet shims for theseparator of FIGS. 1 and 2;

FIG. 5 shows a comparison of the tellurium isotopic spectrum at the 180and 540 positions achieved in the operation of the system of FIGS. 1 and2; and

FIG. 6 shows a comparison of the sulfur isotopic spectrum at the 180 and540 positions achieved in the operation of the system of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings, FIG. 1illustrates generally the structural configuration of the three-stageisotope separator of the present invention. Only the ion source and thebeam boundary plates 20, 20' are shown in FIG. I for the sake ofclarity. The other components of the complete system are schematicallyillustrated in FIGS. 2 and 3A, B, C of the drawings. An ion beam fromthe source 15 is allowed to spiral along the magnetic field in a helicalpath limited by the boundary plates 20, 20 to produce a three-stageseparation after 540 of rotation, and the desired isotopic ions arecollected in a receiver located at the 540 focus in a manner to bedescribed hereinbelow. Although not shown in FIG. I, the ion source, ionreceiver, beam boundary plates, and other essential components areenclosed in a vacuum tank as described hereinafter.

In such an arrangement, the ion source 15 is located off the medianplane close to one tank wall and the isotope receiver or collector 23 atthe 540 location is likewise off center and located close to the othertank wall as can be seen in FIG. 2, as, for example, viewed from thetop. The vacuum tank may be formed, for example, by sidewalls l2 and 13and back and front plates 10 and 11, with the backplate 10 beingprovided with a suitable opening 14 for connection to a vacuum pump, notshown, in a conventional manner. The ion source and ion receiver aremounted from demountable front plate 11 in a conventional manner.Magnets 27 and 28 provide the desired magnetic field B" whose directionis indicated by the arrows in FIG. 2. It should be understood that themagnets 27 and 28 are conventional adjustable electromagnets and thatthese magnets do not necessarily have to be used exterior to the vacuumtank but may be mounted on inner tank liner walls, if such is desired.

The device illustrated in FIG. 2 is not drawn to scale in that the polegap between the magnets 27 and 28 is 24 inches, and the radius of thehelix formed by the boundary plates 20, 20' is also about 24 inches, forexample. The ion source 15 is mechanically positioned in such a manneras to accelerate ions into the field at an angle of 3 20 min. withrespect to one of the magnet poles. To achieve this angle precisely, thesource 15 may have its extraction slit l6 and associated componentsadjustable in position in relation to the magnetic field lines. Theangular position of the source exit slit produces the helical path ofthe ion beam in the magnetic field. The beam progression along themagnetic field from ion source to ion receiver is l35inches.

Another manner in which the ions from the source 15 can be acceleratedinto the magnetic field at a desired angle is to provide magnet shims29, 30 of a shape such as illustrated in FIG. 4 and this would allow thebeam to remain at about 3 20 min. from a perpendicular to the magneticfield for the necessary establishment of the helix. The shims, however,would alter the magnetic field at the ion extraction slit of the ionsource such that the slit may be slightly canted with respect to thefield, if desired, in the vicinity of the shims at an angle of about 15in the same manner as disclosed in the application of William A. Bell,Jr., et al., Ser. No. 885,790, filed Dec. 17, I969, and having a commonassignee with the present application. The canting of the ion extractionslit in this manner provides for a more efficient ion production andextraction from the ion source. The angles shown in FIG. 4 areexaggerated for the sake of clarity.

Referring now to FIGS. 2 and 3A, a baffle 17 is positioned at the 45rotational position of the system in the path of the ion beam and thisbaffle is provided with a suitable opening which restricts the beam tothe desired beam width in both a radial and axial direction. A similarbaffle 7 may be provided at the position of the separator for the samepurpose. Probes, not shown, on baffle 17 are connected to respectivemeters I and 2 as shown in FIG. 3A and are used as an aid in beampositioning. In some operations of the separator it may be desired orpreferred to eliminate the baffle 17 at the 45 position, and, in thiscase, the probes connected to the meters 1 and 2 would be mounted on thebaffle 7 at the 90 position.

At the 180 position of the separator there is provided aconventional-type isotope receiver 22 provided with an ad- 75 justablereceiver plate or baffle l8 and a plurality of isotope collectionpockets, only one such pocket 26 being shown in FIGS. 3A and 3B, for thesake of clarity. These pockets 26 are utilized for beam location,isotope collection, and mass scanning. A meter 3 is connected to thereceiver 26 as an aid in beam location. These pockets are also used formonitoring the separative process with an oscilloscope, not shown, tominimize source-induced oscillations and r. f. picked up by the beamduring the first 180 of rotation.

During a typical separation, all isotopic beams except the desired oneare collected in these receiver pockets at the 180 position as instandard separators. The most desired isotope is allowed to pass freelythrough a slit (e.g., 3/16X2 inch) in an adjustable receiver plate 18and continue to the 360 position as shown in FIG. 3B. At this point, thedesired ions pass through another adjustable receiver plate 19 providedwith a slit (e.g., 3/16 by 2 inch). Probes, not shown, are attached bothabove and below the slit in plate 19 and are connected to respectivemeters 4 and 5 to permit accurate location of this slit relative to thedesired beam which passes freely through this slit. The position of thisslit can be moved in both an axial and radial direction relative to thebeam trajectory.

After the beam enters the last 180 of travel (360-540) it is subjectedto a third mass separative action prior to being received at the 540position in a collector pocket of an isotope receiver 23 as shown inFIG. 3C. This pocket is located behind a defining slot (e.g., 3/16X2inch) in the receiver plate 2] of the receiver 23. The receiver 23 isadjustable in three directions (side-tilt, in-and-out, and up-and-down)and provides essentially limitless choice of pocket position. A meter 6is connected to the receiver 23 as an aid in positioning the slottedreceiver plate 21 with respect to the incoming isotope beam.

It should be understood that the necessary charge vapor for the ionsource 15 of the present invention is supplied to the ion source in aconventional manner. One means for heating the charge material for usein the ion source is disclosed in U.S. Pat. No. 3,115,575, to W. A. Bellet al., to which reference is made. The strength of the magnetic fieldprovided by the magnets 27 and 28 of FIG. 2 is maintained at a selectedvalue in the range from l3,000 gauss depending upon the atomic mass ofthe isotopes to be utilized in the operation thereof. It should beunderstood that even higher magnetic field strengths may be utilized ifnecessary or desired. The pressure which is maintained in the vacuumtank during the operation of the device of FIG. 2 is of the order of 1 0to 10 Torr, for example. The accelerating electrode 24 and deceleratingelectrode 25 associated with the ion source of FIG. 3A provide for thenet acceleration of the ions therefrom up to about 40 kv. The exactvalue is also dependent upon the particular isotopic separation.

In any separator, the relationship of peak ion current to the minimumion current appearing between adjacent peaks (peak-to-valley ratios) atany focal point (180, 360, 540) is indicative of the effectiveness ofthe separation process. Any extraneous ion of another isotopic species,which has been scattered and/or altered in energy, may deviate intrajectory so that it will enter the pocket provided for the collectionof a desired isotope and become a contaminant. Many such contaminantsare stripped off by succeeding stages. This continuous energy spectrumof all isotope species (whether it is due to variation in energy fromthe source, variation in energy and radius from scattering, or to verylow energy ions from the background gas formed by the primary beam)appears at the 180 position as background current. Graphs of ion beam intensities comprised of isotopes of tellurium and sulfur are illustratedin FIGS. 5 and 6, respectively, to show the relative change in thespectrum and background at the normal 180 position and at the 540position.

In FIG. 5, curve 5-A shows the tellurium spectrum at the 540 positionand curve S-B shows the tellurium spectrum at the 180 position. It maybe seen how the background level of extraneous energetic and thermalions was reduced in the 540 position. The best prior "*Te assay from acollection in a conventional 48-inch radius, 180 separator was percent.The

540 separator of the present invention, which has only a 24- inch beamradius, has yielded three samples to date with assays of98. 1, 98.4, and98.95 percent.

In FIG. 6, curve 6-B shows a sulfur spectrum made at the 180 positionand curve 6-A shows one made at the 540 position. The '5 peak wasallowed to go full scale in both positions so the spectral beam qualitycould be observed for this isotope. The spectral beam quality at the 540position (curve 6-A) is clearly superior to the beam quality at the 180position (curve 6-8).

It should be noted and understood that the curves of FIGS. 5 and 6 wereobtained for both positions (180 and 540) by varying and selectivelyadjusting the ion source voltage to different values such that thecomplete spectrum could be 0t tained at each position. This was donesimply to demonstrate that the background level of extraneous energeticand thermal ions was reduced and the spectral beam quality was improvedat the 540 position as compared to the 180 position for all isotopespecies. In the actual operation of the above-described 540 separatordevice, the ion source voltage is maintained at a substantially fixeddesired voltage and the positions of the various baffles or definingslots at the 45 and/or 90, 180, 360, and 540 positions are properlyoriented such that only a desired isotopic species is received at thecollector 23 at the 540 position while all other isotopic species arecollected in the receiver 22 at the 180 position.

Some of the advantages achieved in the operation of the presentinvention in the electromagnetic separation of nuclides are as follows:

1. Eliminating the contaminating effects of variation in beam positionnormally introduced by variation in beam potential. In contrast withsingle-stage separation, the present system provides for three degreesof isolation (0180, 180360, 360-540).

2. Screening out in the additional stages scattered ions not having amass-energy product equaling that of the desired ions.

. Screening out high energy contaminating neutrals formed incharge-exchange processes occurring near the entrance to the collector.

4. Screening out the contaminants resulting from the low energycontinuous spectrum background normally encountered at the receiver.

5. Screening out extraneous ions appearing at the 180 collector slot orslots prior to beam neutralization.

6. Removal of cycloiding low energy ions present in nonneutralizedregions near the receiver.

7. Scattering and collision phenomena in the l80540 region do notintroduce contamination; they only reduce total beam transmission.

8. Output capabilities of the present invention exceed those forsector-type devices because of the present calutronlike configurationand the total beam containment within a strong magnetic field whichrequires only one magnet and magnetic exciter-control system.

9. Electrical isolation of the beam into three segments by the variousbaffles and slits decouples source-induced instabilities and retards thegrowth of beam-induced instabilities.

10. And, finally, an entirely new technique of diagnosis of separatorproblems is provided by the availability of both mechanical and voltagescans at three locations.

It should be noted that a basic limitation in all prior electromagneticseparation devices not equipped with velocity filters is that ions fromthe source, or ions in the beams undergoing collision in the analyzingsegment and acquiring a mass energy product equal to that of the desiredions, reach the isotope collector. Since the defining slots must have afinite size, there is a mass-energy band which is ultimately received.However, the three stages provided in the 540 separator of the presentinvention reduce the size of the finally received mass-energy spectrumto a very narrow value approaching exact matching with the desired beam.Therefore, the isotope separator of the present invention provides animproved system wherein the purity of the desired ions collected at the540 position thereof is substantially improved over that achievable inthe prior art separators, and such a system will be most useful inproducing such high purity separated material where large (calutron)quantities of such separated materials are not required. To obtain thesame purity of the desired separated material in a conventional priorart calutron that is achievable in the present invention with a singleseparation pass (0540), it would be necessary to provide for thereprocessing of separated materials for additional separation runs whenthe prior art devices are utilized. The present invention has beenpracticed in a standard calutron tank and produced of the order of 1/10the separated quantity as a 180 calutron.

It should be understood that charge-exchange type isotope receivers,such as described in US. Pat No. 3,312,849, issued Apr. 4, 1967, to W.A. Bell, Jr., et al., may be used, if desired, at the 540 position inthe present invention to reduce the degree of neutral-particlecontamination, and thus increase the purity of the collected desiredisotope material even further.

It should further be understood that the present invention as describedhereinabove is not limited to the separation of the isotopes oftellurium and sulfur, but may equally be adaptable to provide for theseparation of most other materials. In addition, the above-describedisotope separator is not necessarily limited to the specific dimensionsset forth hereinabove. For example, the magnet gap could be made larger,if desired, to allow an ion beam larger in axial length, and the ionsource may be constructed so as to be mostly away from the beam pathsuch that only the accelerating section thereof would need to be on thehelix perimeter.

The present invention has been described by way of illustration ratherthan limitation and it should be apparent that it is equally applicablein fields other than those described. For example, the invention couldbe used in the field of mass spec trometry.

What is claimed is:

1. An improved electromagnetic isotope separator of the type includingan evacuated tank, means for providing parallel magnetic field linespassing through said tank, an ion source within said tank and providedwith an extraction slit for producing and accelerating ions across saidmagnetic field lines, and an ion receiver within said tank for acceptingbeams of separated isotopes, the improvement which comprises: first ionorienting means associated with said ion source for injecting ions fromsaid ion source across the magnetic field lines at a slight angle fromthe perpendicular thereto so as to cause the ions to follow a spiralpath along said magnetic field lines and form a helical ion beam,slotted bafile means positioned along and in the path of said helicalion beam to eliminate most of any undesired components of said ion beam,second ion orienting means associated with said ion receiver to permitcollection of a desired isotopic ion beam after 540 rotation along saidspiral path from its origin at said ion source, and a pair ofspaced-apart boundary plates mounted between said ion source and saidreceiver and positioned in such a manner that said desired ions spiralin a helical path between said boundary plates from said ion source tosaid receiver, said slotted baffle means including a first adjustablebaffle plate provided with an elongated slit and positioned between saidboundary plates at the 45 position from said ion source for restrictingthe ion beam from said source to a desired beam width, a secondadjustable baffle plate provided with an elongated slit and positionedbetween said boundary plates at the 180 position from said ion sourcefor permitting a desired isotopic ion beam to pass freely through theslit thereof, a third adjustable baffle plate provided with an elongatedslit and positioned between said boundary plates at the 360 positionfrom said ion source such that said desired isotopic beam passes freelythrough said slit of said third baffle plate, and a plurality of isotopereceiver pockets mounted on said second adjustable baffle plate at the180 position, said second baffle plate being further provided withreceiver slits for each of said plurality of receiver pockets and saidsecond ion orienting means comprising an adjustable receiver plate whichis provided with an elongated slit through which said desired isotopicbeam is adapted to pass and be collected by said ion receiver, wherebyall isotopic beams except said desired isotopic ion beam are collectedin said plurality of receiver pockets at the 180 position, and saiddesired isotopic ion beam collected in a 540 ion receiver hassubstantially improved purity.

2. The separator set forth in claim 1, wherein said first ion orientingmeans is adapted to be mechanically positioned to provide for saidslight angle.

3. The separator set forth in claim 1, wherein said first ion orientingmeans comprises a pair of magnet shims which are mounted on oppositesides of the extraction slit of said ion source to provide for saidslight angle.

4. The separator set forth in claim 3, wherein said ion sourceextraction slit is canted at an angle of about 15 with respect to thedirection of the magnetic field provided by said magnet shims.

5. The separator set forth in claim 1, wherein said magnetic field is ofa selected value in the range from to 3,000 gauss, and said tank isevacuated to a pressure of a selected value in the range from 10 to 10Torr.

6. The separator set forth in claim 5, wherein said slight angle isabout 3 20 min.

7. The separator set forth in claim 6, wherein said first ion orientingmeans comprises a pair of magnet shims which are mounted on oppositesides of the extraction slit of said ion source to provide for saidslight angle.

8. The separator set forth in claim 7, wherein said ion sourceextraction slit is canted at an angle of about l.5 with respect to thedirection of the magnetic field provided by said magnet shims.

9. The separator set forth in claim 8, and further including a pair ofseparator probes connected to separate meters in the vicinity of each ofsaid 45 and 360 baffle plates and a separate meter connected to areceiver pocket at each of said 180 and 540 positions, said metersfunctioning as an aid in the proper positioning of said baffle platesand said receiver plate with respect to the path of said desiredisotopic beam.

UNITED STATES PATENT OFFICE CETIFIQATE or QQRREQ'H Patent No. 3 7 D dMarch 14, 1972 Inventor(s) WiHiam A. Be] 1 JY. et a] It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 22, for "throughout" read ---throughput-- Column 6, line53 for "separator" read ---separate-- Signed and sealed this 22nd day ofAugust 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBEIftT QOTTSCHALK Attesting Officer Commissionerof Patents USCOMM-DC 60376-P69 u.s. GOVERNMENT PRINTING OFFICE: 19690-366-33A

2. The separator set forth in claim 1, wherein said first ion orientingmeans is adapted to be mechanically positioned to provide for saidslight angle.
 3. The separator set forth in claim 1, wherein said firstion orienting means comprises a pair of magnet shims which are mountedon opposite sides of the extraction slit of said ion source to providefor said slight angle.
 4. The separator set forth in claim 3, whereinsaid ion source extraction slit is canted at an angle of about 1.5* withrespect to the direcTion of the magnetic field provided by said magnetshims.
 5. The separator set forth in claim 1, wherein said magneticfield is of a selected value in the range from 100 to 3,000 gauss, andsaid tank is evacuated to a pressure of a selected value in the rangefrom 10 5 to 10 6 Torr.
 6. The separator set forth in claim 5, whereinsaid slight angle is about 3* 20 min.
 7. The separator set forth inclaim 6, wherein said first ion orienting means comprises a pair ofmagnet shims which are mounted on opposite sides of the extraction slitof said ion source to provide for said slight angle.
 8. The separatorset forth in claim 7, wherein said ion source extraction slit is cantedat an angle of about 1.5* with respect to the direction of the magneticfield provided by said magnet shims.
 9. The separator set forth in claim8, and further including a pair of separator probes connected toseparate meters in the vicinity of each of said 45* and 360* baffleplates and a separate meter connected to a receiver pocket at each ofsaid 180* and 540* positions, said meters functioning as an aid in theproper positioning of said baffle plates and said receiver plate withrespect to the path of said desired isotopic beam.